1. Field of the Invention
This invention generally relates to signal communications and, more particularly, to a system and method for controlling polarization in a polarization multiplexed network using a multi-channel Optical Transport Network (OTN) protocol.
2. Description of the Related Art
100 gigabit per second (Gbps) OTN protocols are being developed where the OTN streams are carried in a “parallel fashion” at a bit-rate greater than 100 Gbps. Until recently, optical signals have been carried using modulations techniques carrying 1 bit per symbol. However, the need for faster signal speeds means that return to zero (RZ), non-return to zero (NRZ), and phase-shift keying (PSK) modulation techniques (1 bit per symbol) are no longer suitable.
Two complex envelope signals, such as 2 bits/sym quadrature phase-shift keying (QPSK) signals, can be formed to coexist in a medium such as fiber via polarization multiplexer, one in an X polarization and the other in the polarization. This bandwidth efficiency of such a composite signal is greater. When this composite signal travels in a medium such as a fiber, the state of polarization, i.e. the definition of X and Y polarizations, may not be the same for the receiver as the transmitter.
FIG. 1 is a schematic diagram depicting a system for the transmission of high speed signals (40/100 Gbps and above) using multiplexed orthogonal optical polarization, with 2 bits per symbol per polarization, for a total of 4 bits/symbol (prior art). Mach-Zehnder modulators (MZMs) convert electrical signals to optical signals, and phase shift is introduced between the I and Q signal paths. Any skew (differential time delay) experienced in the modulation of the transmitted signal is translated to the receiver, making the recovery of a serial stream from parallel streams difficult. Alternately but not shown, phase shift may be introduced before the electrical signals are converted to optical signals. After transmission, polarization errors may also be introduced.
It would be advantageous if technique could be developed to minimize errors due to changes in polarization in the transmission of composite polarized signals.